How does optical fiber carry data?

Inside the medium

Light travels through a carefully built guide.

An optical fiber has a glass core surrounded by cladding with a different refractive index. The cladding keeps light confined to the core, while a protective coating and jacket protect the glass from moisture, bending, and mechanical stress.

A transceiver converts electrical bits into modulated light at the transmitting end. The receiver detects the light and converts it back into an electrical signal. The fiber carries the optical signal; it does not interpret IP, Ethernet, or the application data by itself.

Guide the light

The refractive index determines how light changes direction and speed in a material.

The refractive index n compares the speed of light in vacuum with its phase velocity in a material: n = c / v. A higher index means that light propagates more slowly in that medium and generally bends toward the normal when it enters from a lower-index medium.

At the boundary between two materials, Snell's law relates the incident and refracted angles: n1 sin(theta1) = n2 sin(theta2). In an optical fiber, the core has a slightly higher refractive index than the cladding. When the angle is large enough, the light reaches total internal reflection and remains guided inside the core.

Encode and recover

How do bits become light, then become bits again?

At the transmitter, an electrical data stream controls a laser or LED driver. In the simplest form, the driver varies the optical power: light above a decision threshold can represent one symbol and light below it another. This is a physical representation of the data, not a change to the data's meaning.

At the receiver, a photodiode converts incoming photons into a small photocurrent. An amplifier increases the signal, while timing recovery and a decision circuit or DSP estimate the transmitted symbols and output an electrical bit stream.

Choose the propagation geometry

Single-mode and multimode solve different distance problems.

Preserve the pulse shape

Chromatic dispersion makes different wavelengths arrive at different times.

A transmitter does not always emit one perfectly single-frequency wavelength. Because the fiber's propagation speed varies with wavelength, the spectral components of one optical pulse can travel at slightly different speeds. The pulse spreads as it moves along the fiber.

Chromatic dispersion becomes more restrictive as the distance, symbol rate, or optical bandwidth increases. Engineers can reduce its impact with a narrow-linewidth source, a suitable wavelength such as the low-dispersion region near 1310 nm, dispersion-managed fiber, compensating modules, or coherent receivers with digital signal processing.

Match the optical components

Wavelength is part of the link design.

Common optical systems use windows around 850 nm, 1310 nm, or 1550 nm. The chosen wavelength affects attenuation, dispersion, the transmitter, the receiver, and which fiber and optic combinations are compatible. A transmission window is a wavelength range where the fiber and the available components provide a useful balance of loss and dispersion.

The single-mode spectrum is often described with bands: O-band around 1260–1360 nm, E-band around 1360–1460 nm, S-band around 1460–1530 nm, C-band around 1530–1565 nm, and L-band around 1565–1625 nm. These ranges are engineering conventions, not separate kinds of light.

Share an access fiber

PON lets one provider fiber serve several customers.

A Passive Optical Network uses an OLT at the provider, passive optical splitters in the optical distribution network, and an ONT/ONU at the customer. Downstream light can reach several customers; upstream transmission is scheduled in time slots so customers do not transmit over one another.

Place the fiber endpoint

FTTx names the last destination of the fiber.

FTTx is an access-network naming family. The final letter or letters indicate where the optical fiber ends; the remaining distance may use Ethernet, coaxial cable, twisted pair, or another medium. The exact meaning of FTTP can vary: many operators use it as an umbrella for fiber to a premise, while others use it almost interchangeably with FTTH.

Check the physical margin

The optical budget explains whether a link can close.

A simplified link budget compares the available transmitter-to-receiver power margin with the losses in the path. Fiber attenuation, connectors, splices, bends, and other passive components consume that margin.

Too little received power can produce errors or a link that never comes up. Excessive power can also overload a receiver in some systems, so optical engineering checks both the lower and upper operating limits.

Read the interface name

Ethernet optics pair a rate with a fiber family.

Modules such as SFP, SFP+, and QSFP describe transceiver form factors and families, not a universal guarantee that any module works with any fiber. Always match the optic's rate, wavelength, fiber type, connector, reach, and vendor requirements.